This invention relates to multiple pump fluid delivery systems, such as for example, systems used to deliver multiple different solvents to columns in gradient elution liquid chromatography or to pressure vessels in fluid extraction or systems used to switch the outlet flow from one syringe pump to another to provide continuous flow without interruption or fluid disturbance.
In one class of multiple pump fluid delivery system, an effort is made to control the pressure of the fluids delivered to a vessel or column and in some such pressure controlled systems to also control the composition such as for example the ratio of the different fluids. In some such systems using controlled pressure and composition, the flow rate is controlled from each of a plurality of different pumps. The pressure is measured by transducers and one or more signals fed back to control the pumping motors and thus to maintain the proper flow rate for the desired composition and the proper pressure. These systems are of particular significance in supercritical fluid chromatography and supercritical fluid extraction systems using multiple solvents.
In one prior art system of this type, disclosed in U.S. Pat. No. 5,071,562, two syringe pumps for two different fluids are independently controlled for flow rate by a gradient controller and the pressures are equalized between the two pumps by a mechanical mixer which permits only the first or primary fluid to flow at the programmed rate while the second pump under program control builds up pressure to equal the pressure of the first pump so that a uniform pressure is applied to the fluids mixed in the mixer as they flow to the chromatographic column. The pressure equalizing mechanism is combined in mechanical structure with the mixer.
This system is relatively complex and suitable for only two pumps. It is known to control instability using feedback circuitry. One such system which controls transient instability in a single fluid flow system including a syringe pump is disclosed in U.S. Pat. No. 4,043,906 to Helmet. This system utilizes a multiple degree feedback system to provide adequate damping against transients. There is no disclosure of how to adapt this system to a multiple solvent system which controls pressure so as to maintain stability while at the same time providing the proper composition. This is a particularly significant problem when using supercritical fluids because of their compressibility.
In another prior art system, disclosed in U.S. Pat. No. 4,347,131 filed in the name of Brownlee, a continuously flowing pumping system with mechanically operated valves is disclosed. This system uses two syringe pumps which operate reciprocally. However, there is no disclosure on how the valves or other parts of the system may be automatically operated to transfer flow from one pump to another without disturbance. A similar pump has been sold commercially in the United States by Chromatramix, Inc.
Generally, pressure transducers are not well enough matched in accuracy to measure pressure at multiple points in the fluid system to be used to control valves automatically for the prevention of the transfer of fluid flow from one pump to the next without disturbance.